Tressa Montalvo places her hand on newborn son Blaine at the Women's Hospital of Texas.

Tressa Montalvo places her hand on newborn son Blaine at the...

NICU nurse Lynn Austin takes care of Ace Montalvo at The Women's Hospital of Texas on Monday, Feb. 18, 2013, in Houston. Ace Montalvo is one of four babies born to Tessa Montalvo, who gave birth to two sets of identical twins on Valentine's Day.

Photo: Brett Coomer, Houston Chronicle

NICU nurse Lynn Austin takes care of Ace Montalvo at The Women's...

Cash Montalvo is shown in the NICU at The Women's Hospital of Texas on Monday, Feb. 18, 2013, in Houston. Tessa Montalvo gave birth to two sets of identical twins on Valentine's Day, born at 31 weeks.

Photo: Brett Coomer, Houston Chronicle

Cash Montalvo is shown in the NICU at The Women's Hospital of Texas...

Dylan Montalvo is shown in the NICU at The Women's Hospital of Texas on Monday, Feb. 18, 2013, in Houston. Tessa Montalvo gave birth to two sets of identical twins on Valentine's Day, born at 31 weeks.

Photo: Brett Coomer, Houston Chronicle

Dylan Montalvo is shown in the NICU at The Women's Hospital of...

Blaine Montalvo is shown in the NICU at The Women's Hospital of Texas on Monday, Feb. 18, 2013, in Houston. Tessa Montalvo gave birth to two sets of identical twins on Valentine's Day, born at 31 weeks.

Photo: Brett Coomer, Houston Chronicle

Blaine Montalvo is shown in the NICU at The Women's Hospital of...

Ace Montalvo is shown in the NICU at The Women's Hospital of Texas on Monday, Feb. 18, 2013, in Houston. Tessa Montalvo gave birth to two sets of identical twins on Valentine's Day, born at 31 weeks.

HOUSTON — Tressa Montalvo loved growing up in a big family with six siblings, but she never expected her own to become so big, so fast.

The 36-year-old Houston native gave birth at the Woman's Hospital of Texas to a rare form of quadruplets: two sets of identical twins.

Unlike quadruplets who split from one egg and grow in one placenta, these four boys grew in pairs within two placentas for 31 weeks before Tressa had a complication-free cesarean section on Valentine's Day.

The arrival of Ace and Blaine then Cash and Dylan a minute later was even more unusual because the Montalvos didn't take fertility drugs or use in-vitro fertilization, which have contributed to a 76 percent increase in multiple births nationwide between 1980 and 2009, according to the Centers for Disease Control and Prevention.

According to previous reports, Dr. James Grifo, director of the NYU Fertility Center, has said the odds of having two sets of identical twins at the same time are 1 in 10,000 with in-vitro and much rarer with natural conception.

It happened in San Antonio last month when Shontae Minor, 22, and Khristian Rohena, 34, welcomed Yandel, Yariel, Orion and Ryan Rohena, two sets of twin boys conceived naturally.

Yandel died about a week after being born, and Ryan died Feb. 7 and was buried a week later, on the day the Houston quadruplets were born.

On Monday, Manuel, 43, pushed his wife's wheelchair into the neonatal intensive care unit at the Houston hospital, their 2-year-old son Memphis holding Mickey Mouse as he sat on his mother's lap.

Manuel picked up his oldest son and carried him to the nearest sealed crib.